Method of fabricating masking sheets



June 22, 1965 R. DAHLBERG METHOD OF FABRICATING MASKING SHEETS Filed June 19, 1961 INVENTOR REINHARD DAHLBERG ATTORNEY United States Patent O ens-e772;

laiETHQE) E FABRKIATING MASKING SHEETS Reinhard Dahlberg, .Fre'ihurg, Breisgau, Germanmassigner to Clevite Corporatiosnacorporation of Ohio "Filed June 1%, 1961, Ser. No. 118,172" 'Claims priority, application Germany, rune 18,1969,

318,298 6 Claims. (Cl. 156-62) is essentialin the-fabrication. of; transistors, particularly those suitable =for,-;.operation -at :highxfrequeneies, -to torm the respective contiguous layersof diiferent-conductivity types as thin as possible andto. control the size, shape and-location of. rectifying junctions with great -accuracy.- =-"Fo:this

end :special fabricating techniques have developed which yield semiconductor devices such as the.so-called-?mesa"7 transistornand which utilizealloying vaporizing and! or diffusion of conductivity-typedeterminants.in-.the formation of the-junctions .--It1.mass. production -a .large number ot-devices-imay-be formed ona singlelarge wafer of-the semiconductor material which is subsequently subdividedto obtain-individoal elementsh-alnt order to :enable control ofthe size;'lo- Cation andishape. ofithe regions to which the conductivitytype. determina-ntss are applied masks arewidely:used..-,=.-.it m: F..-

While, simple in concept and structure; the. fabrication of suitable masks is fraught with difficulties and continues to be an,important-problem-inthe; mass production of transistors and. other semiconductor devices .of this type.

on the wafer, vaporizing ..'.l he masks-usually take the-form of ;a thinflat-Esheet having a. griddike. structure, i-.e,, perforated with :a large number of small closelysspacedholes. -.The simplest and perhapsth e rnost; obviousmethod of preparingsuch -a mask; on iste implyp nchi s a p iholes; in a, sheet. ofmfiial .foil, in.. the desired arrangement '.-The manifest, simplicity of; this..,appr0aqh,- :howevenis; wholly sup erficial andis quickly dispelled by-aconsidertxtiQn-of the dimensional requirements involved.

.H iQ freaus s aoperatten, emi ond r I devices must be; constructed ,to. extremely ,small dimension; and closely:held tolerances. Consequently, preparing. -.a suit,-

able. mask .in this manner ,would require .punchingholes of; 25 by l oi) microns in, toilabout 50- microns-thick.

e constructionotpunches and dies forsuch-am-oper-az ible, .Tp circumvent .thistdifficultyr it Len proposed toutilize doublefoils containing simultaneously punched holes-of a;relatively.large,r;size,,e.g;, 300 by SQQmierons ended-setting the respectivezfoils so hat-the 1.

occluded to thedesircdsize. Another proposed solutionto the problem hasbeen the use of electric dischargesto form the tiny perforations.

method is,- .of cours e,.very costly.. Boththe mechanicaland electrical methodsof perforation have the additional disadvantage of subjecting the material to mechanical or :therrnal effects, introducing stresses which cause the masks to warp or buckle; consequently, the masks do not lie flat in service, resulting in relatively large pe rtures are, partially. outot. register-and are I .su-liiciently sharp and accurate shaded or partially masked regions detracting significantly from the accuracy of the pattern produced.

Another known approach to the fabrication of vapor,- izingmasks utilizes a ,photolithographic technique com bined with etching. The metal foil which is to form the mask: is-coatedwith aphotosensitive layer and exposed to photoe-ltective illumination passing-through a transparency bearing the desired pattern in the form of selected opaquetand transparent'areasw. The photosensitive coating. is such that, by exposure. to the photoetiective illumina: tion, .it is renderedresistant to .a particular solvent ellecs tiveaon the unexposed coating.v .The developmentlof thefoil, therefore, is accomplished, by appiicationof the solvent which removes the photosensitivecoating from the foil except in theregions exposed to the photoefiective illumination.- .2

After development the foil is: etched to rform apertures inrthe regions of the-toil devoid of the exposed and developed coating: The difiicultyzwith this technique-re: sides in the preparation'of thepattern on the transparency through which the light-sensitized foil :is exposed.- By enlarging: the male approximately zten; times,- :it is @possible to draw conveniently the-pattern ufor. the; transparency but this entails subsequent photographic reduction of the pate Etern tort-he. desired size.-.-; This isusually; done by projecting the drawn i-rn'agezon photographic. film. --However,- due to Ethe distortion in t the optical system i and limits .ofthe resolving p0- 'er.- of-therfilm, it isnot possible. 10.!Obl2ll1 facsimiles of P the original (drawn) .pattern. 2.:=- u- 1.5-;

:::'l hese* dificulties can :be :lessened. by minimizing the amount ofzreductiohrequired;but-this leads to extraordina ry ineasures'for drawing the original 'pattern to a smaller scale hus, :forexamplepwiththe aid ofafprecision engraving machine a pattern can be etched on at-blackenedmetal: platerto! a-suficientlysmall scale-that only a 14:1 optical: reduction" is required.- "Even' I this, however, does-not adequately solve the problem because the. en:- gr'aving machine leaves. grit particles :xvhich, in. conjunction=with eventhe-lowenratio of optieal-reduction,-oauses distortion and-blurred-latches; r lt is thefundamental generalobject'of the/present invention to avoid entirely or substantially mitigate the problemsaiid disadvantages of' the prior art as'sun'imarized abovel I: "Am'OI6' specific-object is to provide novel methods of -forming vaporizing masks-in which the desired pattern is drawn fuilsca'lein the original and therefore doesnot requireoptical'reduetion.- z is z-Another object- -is 'the provision of forming vaporizing masks stressesinthe mask. Y 'Acorollary of the preceding object is the'provisionof vaporizing masks Whiehdie'fiat and in intimatewontact withthe surfaces tovv'hichthey are appliedl further object is the provision of'vaporizingmasks andthe-like characterized-bye high-degree of precision in the location, dimensions, and' configuration *of the'elemental'units of-itspatternr' These-and additional-objectsor the invention 'are" fulfilled by methods of fabricating-vaporizing"inasks 'am cording to the present invention which-include providing a fiat plate of relatively hard-materialhaving-on a major surface a coating of relatively *softer material; mech-an ica'lly inscrlbingon the' plate by selective localiz'ed'removal of the coating a' precise patternof'fine'intersecting line's defined'and characterized by the absence of coating material from the'major surface; and employing the inscribed plate 'as a pattern 'for making perforations in a thin metallic sheet. i

Further objects of the invention, its advanta of improved methods which do not introduce scope G and the manner in which it may be practiced will be more readily apparent to persons conversant with the art from the following description and subjoined claims taken in conjunction with the annexed drawing in which,

FIGURE 1 is a diagrammatic view, in perspective, of an exemplary form of apparatus which may be employed for performance of the method contemplated by the present invention; and

FIGURE 2 is a plan view showing, on a greatly enlarged scale, a pattern produced in accordance with the present invention.

Referring first to FIGURE 1, reference numeral designates generally an optical stage of typical and wholly conventional construction. Briefiy stated, such an apparatus comprises several flat plates superposed and arranged for independent translatory displacement or rotation relative to one another.

The stage shown in FIGURE 1 comprises five plates designated respectively, from top to bottom, 12, 14, 16, 18 and 20. Plates 14 and 16 are rotatable with respect to each other about vertical axes and, while included because they are among the usual and important components of a typical optical stage, they are not in principal required in the performance of the method contemplated by the invention albeit they may be used, if desired, to

increase precision in a manner which will be obvious from the following description.

Plates 18 and 20 are arranged for displacement in parallel planes along respective, mutually perpendicular axes, displacement being effected and controlled with a high degree of accuracy by means of individual micrometer adjustment screws 22 and 24. On the uppermost plate 12 there is fixed by any suitable means such as clamps 26, a plate 28 of relatively hard material, hereinafter described with greater particularity. Plate 28 is disposed and maintained by clamps 26 in a plane parallel to the planes and axes of the adjustment of plates 18 and 20.

Optical stage 10 is disposed, with respect to a vertically mounted scribe assembly 30, so that the working tip of a scribe 32 contacts the upper surface of plate 28. Scribe 32 is held in a suitable chuck 34 slidably mounted in a guide member 36 which constrains the movement of the chuck to linear displacement along a vertical axis and, therefore, perpendicular to the planes of the various plates of optical stage 10.

Chuck 34 is spring pressed, by any suitable means not shown, so that the working tip of scribe 32 is normally urged into contact with the upper surface of plate 28. The working tip of the scribe is fiat and co-planar with plate 85. In the event that the axis of scribe 32 is at an acute angle to the plane of plate 28 and optical stage 10, the tip of the scribe would possess a corresponding angle so as to make surf-ace contact, rather than point or edge contact, with the surface on which it bears. The factors governing the size of the scribe tip will become apparent as the present description proceeds.

Prior to installation on optical stage 10, plate 28 has applied to at least one major surface, uppermost when mounted on the stage, a thin surface layer or coating 38 of a material softer than the plate itself. In accordance with particular features of the invention the respective materials of coating 38 and plate 28 are closely interrelated; the material of the plate may be considered as falling in one of two principal categories: (1) hard transparent materials such as glass or (2) opaque metallic substances such as tool steel and similar hard steel alloys.

Additional requirements for the various materials for both plate 28 and coating 38 will be more readily understood and appreciated in view of a description of these screws 22-, 24 is operated to displace the associated plate 13 or 28 of optical stage 19. As a result of this displacement, scribe 32 traces a line on the plate by local removal of the coating. Having traced such a line across the entire coated surface of plate 28, the other micrometer screw is adjusted to displace the plate a predetermined amount in a direction perpendicular to the line traced thereon by the scribe and, thereafter, the first micrometer screw operated once again to trace a second line precisely parallel to the first.

From the foregoing it will be understood that by alternate and repeitive manipulation of the micrometer screws a precise pattern of fine intersecting lines can be traced on plate 28.

Such a pattern is illustrated on a highly exaggerated scale in FIGURE 2 where it appears as a grid formed by a series of parallel lines 4-0 formed by removal of coating 38 intersected at right angles by a second series of parallel lines 42. The lines delineate on the surface a pattern composed of uniform segments 38' of unremoved coating. The size, shape and location of segments 38 correspond to those of the perforations in the mask being fabricated.

From the foregoing it will be appreciated that the plate 28 must be of a very hard material, whether metallic and opaque or transparent, and coating 38, in addition to being softer, must adhere to the plate so that, in the process of engraving the pattern, the lines scribed are well-defined with sharp edges and the unremoved coating segments do not become detached wholly or partially during subsequent processing.

In accordance with the particular feature of the present invention involving the use of glass or like transparent material for plate 28, satisfactory results have been obtained by forming coating 38 by vapor deposition of an antimony or tin. Vapor deposited tin-bismuth and tingold alloys are also suitable.

Prior to application of coating 38 to the glass plate it is advantageous to apply a film (not shown) of tough, translucent paint to the surface which is to be coated. Such a coating enhances the characteristics of the sliding contact between the scribe and the upper surface of plate 28.

With the pattern prepared on glass and defined in the form of effectively transparent and opaque areas, the pattern is conveniently duplicated by a photographic reproduction process, viz., contact printing on photographic film which, in the interests of high resolution, should be of a type having a thin, fine-grained, emulsion. The plate is placed in direct contact with a sheet of the photographic film which -is then exposed to photoetfective illumination through the plate. The latent image thus formed is developed and fixed by any suitable chemical treatment.

The developed film pattern is then placed in direct intimate surface contact with a suitable metal foil, e.g., molybdenum, having a photosensitive coating. The photosensitive coating of the metal foil is exposed to illumination passing through the film pattern and the foil subsequently subjected to a development" treatment which causes the unexposed portions only of the photosensitive coating to be removed leaving a positive reproduction of the pattern on the foil.

At this stage the metal foil is subjected to a nickel plating treatment, conveniently by immersion in an electroless nickel plating bath, and a nickel coating thus deposited on the areas of the foil only which are devoid of the photosensitive coating. The nickel plating procedure is continued until a cohesive nickel foil is formed containing perforations corresponding in size, shape and location to the regions of the metal foil on which the photosensitive paint is retained. The nickel foil thus formed is then loosened and separated from the metal foil and constitutes a vaporizing or alloying mask.

--Where the original pattern is engraved on a plate of tool 5,. steel or simiiar metal the procedure is somewhat different. The material utilized for coating'QiSniay be a 'so fter metal b'ut'pre rably is resin. Especially suited'to this purpose s of unsaturated hydrocarbon f su'ch as polyindene nol resinsTPrior jt'o application of'theresin co'a ng, underco atin'g of chromium pre'ferablyis ap plie th suiface iof the s tee llplate. The' chromium layer facilitates of the scribe ov'er'Qthc surface of the plate sashes additional'advantages which will beco p p p X 'th" descriptionpr oc'eeds. -'lheipattern 1's eng 'i'rithe manneralready described hef nab I casethepatternistle'finedbyrespec tive region provided with n resinouscoating andregions where the resin h as been removed.

The plate is subjected 'toa nick p ating treatment in the same mannei as the giass pla te whereby a nickel coating is deposited on the chromium areas from-which resin/has been removed, withzthe result that a thin :IllCkBl foil i'sxproduced, which is a positive treplicaof the original pattern (i. e., having solid regionscorresponding to the lineslaor grooves of the pattern-and perforations cor responding in shape, area and location to segments of unremoved resin :on the metal-=plate.) I? Particularly where the steel plate has been chromium-plated, the nickel foil thus formed can be easily peeled off.

The nickel foil can be used directly as a vaporizing mask at this stage, if desired. However, the foil produced in this manner is usually very thin and easily torn; for this reason it is preferable to use this first foil as a transparency to make heavier masks in the manner employed in conjunction with the case where the original pattern is scribed on a glass plate. Specifically, the first thin nickel foil is used to expose a heavier metal foil of molybdenum, for example, having a light-sensitive coating. This is developed and utilized for the fabrication of masks in the same manner as described hereinabove.

Masks prepared according to the present invention are characterized by high precision due to the elimination of errors introduced by optical distortion and blurring of lines. This is reflected in improved uniformity in electrie What is claimed and desired to be secured by United States Letters Patent is:

1. A method of fabricating thin metallic sheets containing minute, closely spaced perforations, which includes; providing a plate of relatively hard, effectively translucent material having on a major surface a thin effectively opaque coating of relatively softer material; engraving said surface so as to selectively remove portions of said coating in a precise predetermined pattern of fine intersecting lines; providing a sheet of metal foil having a photosensitive coating; exposing the photosensitive coating of said metal foil to photoeffective illumina tion through said pattern to produce an image of said pattern on the photosensitive coating without change of scale; subjecting the exposed metal foil to a development treatment effective to cause removal of unexposed portions only of the photosensitive coating; and thereafter subjecting the thin metal sheet to a metal plating treatment efiective to deposit a metal layer on areas of the thin metal sheet free of the photosensitive coating to form a metal foil containing perforations corresponding in size, configuration and location to areas of the photosensitive coating exposed and remaining on said thin metal sheets and removing the metal foil from said thin metal sheet.

2. A method of fabricating thin metallic sheets con- .6 taining minute, closely-spaced perforations, which includes: providing'a plate of relatively hard, effectively ranslucentm terialist/m on amajoi surface 'a thin effectively opaque coating of relatively s'ofter' material; engraving said surface so as to s'lectively r'emoveportions of Said coating in a precise predetermined pattern bf fine intersectinglines; providing'a sheet of metal'foil'havirig a photosensitive coating; exposingthe photosensitivecoat lag of said metal an to photoeffective illumination throu h said patterntoprodtice'an image of said patternori the photo'sehsitivefoating without chzinge of scale; "subjecting the exposed 'rnetalfo'il re a "development treatment effec tive to cause removal of unexposed portionsbrily o'f the photosensitive coatingjfther e'after' subjectingthe'tii "metal sheet to a nickel platirilg'tr'eatriient etfectiv'td'depfsit a. nickel layer on'a're'as' of the-meanes free of the photo'- s'ens'itiv'e" eoa'tingto' form anick'el foil" containing perforatio'ris" correspondingfin' 'si z' c n'figtiratior'i ar'id' ldcaf tion to areas of the photosens'rtiv coating sense-d an remainingenisaid'thin met-a1 sfh t; 11 an viiig-the aoii-ffor'nsaid' thinir'ne'talsheet. q 'ting' thi'ri metallic "sheets" on tai'rting minute, moiety-spasm perforations, wiiic'h 1ncludes: providing a plate of relatively hard, effectively translucent material having on a major surface a thin effectively opaque coating of relatively soften material;

engraving said surface so as to selectively remove portions of said coating in a precise predetermined pattern of fine intersecting lines; providing a sheet of photographic film having a thin, fine-grained photosensitive emulsion; placing said film sheet and plate in intimate surface contact and exposing said emulsion to photoelfective illumination passing through said film so as to form a latent image of said pattern on said film; developing the exposed film; providing a thin sheet of metal having a photosensitive coating; placing the developed film in surface contact with the photosensitive coating of said thin metal sheet and exposing said coating to photoeffective illumination through said developed film; subjecting the exposed metal foil to a development treatment effective to cause removal of unexposed portions only of the photosensitive coating; thereafter subjecting the thin metal sheet to a nickel plating treatment effective to deposit a nickel layer on areas of the metal sheet free of the photosensitive coating to form a nickel foil containing perforations corresponding in size, configuration and location to areas of the photosensitive coating exposed and remaining on said thin metal sheet; and removing the nickel foil from said thin metal sheet.

4. A method of fabricating thin metallic sheets containing minute, closely-spaced perforations, which includes: providing a glass plate; vapor depositing on a major surface of said glass plate a thin metal coating selected from the group consisting of antimony, tin, tinbismuth alloys and tin-gold alloys; engraving said surface so as to selectively remove portions of said coating in a precise pattern of fine intersecting closely-spaced lines; providing a sheet of photographic film having a thin, finegrained photosensitive emulsion; placing said film and plate in intimate surface contact and exposing said emu1 sion to photoeffective illumination passing through said film so as to form a latent image of said pattern on said film; developing the image on the exposed film; providing a sheet of molybdenum foil having a photosensitive coating; placing the developed film in surface contact with the photosensitive coating of said molybdenum foil and exposing said coating to photoeifective illumination through said developed film; subjecting the exposed molybdenum foil to a development treatment effective to cause removal of unexposed portions only of the photosensitive coating; thereafter subjecting the molybdenum foil to a nickel plating treatment effective to deposit a cohesive nickel layer on areas of the foil free of the photosensitive coating to form a nickel foil containing perfora- 7 tions corresponding in size, configuration and location to areas of the photosensitive coating exposed and remaining on said molybdenum foil; and separating the nickel foil from the molybdenum foil.

5. A method of fabricating very thin metallic sheets containing minute, closely-spaced perforations, which includes: providing a hard metal plate; applying a resinous coating to a major surface of said plate; mechanically inscribing on said plate, by selective removal of said resinous coating, a precise pattern of fine intersecting lines defined and characterized by the local absence of said resinous coating from said major surface; and subjecting the plate, so inscribed, to a nickel plating treatment effective to deposit a layer of nickel only on areas of said surface free of the resinous coating with concomitant formation of a nickel foil containing apertures corresponding in size, configuration and location to areas of said major sur face retaining said resinous coating.

6. A method of fabricating very thin metallic sheets containing minute, closely-spaced perforations, which includes: providing a hard steel plate, chromium-plated; applying to a major surface of said plate a coating of a resin selected from the group consisting of polyindene and terpene phenol resins; mechanically inscribing on said steel plate, by selective removal of said resin coating, a precise pattern of fine intersecting lines defined and characterized by the local absence of said resinous coating from said major surface; and subjecting the plate, so inscribed to a nickel plating treatment effective to deposit a layer of nickel only on areas of said surface free of the resinous coating to form a nickel foil containing apertures corresponding in size, configuration and location to areas of said major surface retaining said resinous coating; and removing the nickel foil from the steel plate.

References Cited by the Examiner UNITED STATES PATENTS 2,019,590 11/35 Westra 41-25 2,333,251 11/43 Huggins c 156-12 2,346,231 4/44 Ormond 15612 XR 2,738,730 3/56 Boyajean 41-24 XR 2,762,149 9/56 Mears 96-36 EARL M. BERGERT, Primary Examiner. 

1. A METHOD OF FABRICATING THIN METALLIC SHEETS CONTAINING MINUTE, CLOSELY SPACED PERFORATIONS, WHICH INCLUDES; PROVIDING A PLATE OF RELATIVELY HARD, EFFECTIVELY TRANSLUCENT MATERIAL HAVING ON A MAJOR SURFACE A THIN EFFECTIVELY OPAQUE COATING OF RELATIVELY SOFTER MATERIAL; ENGRAVING SAID SURFACE SO AS TO SELECTIVELY REMOVE PORTIONS OF SAID COATING IN A PRECISE PREDETERMINED PATTERN OF FINE INTERSECTING LINES; PROVIDING A SHEET OF METAL FOIL HAVING A PHOTOSENSITIVE COATING; EXPOSING THE PHOTOSENSITIVE COATING OF SAID METAL FOIL TO PHOTOEFFECTIVE ILLIMINATION THROUGH SAID PATTERN TO PRODUCE AN IMAGE OF SAID PATTERN ON THE PHOTOSENSITIVE COATING WITHOUT CHANGE OF SCALE; SUBJECTING THE EXPOSED METAL FORIL TO A DEVELOPMENT 